1. Field of the Invention
The present invention relates to brush seals for sealing high pressure fluid areas from low pressure fluid areas, the sealed fluids being gas or liquid.
2. Description of the Related Art
Over the last decade, brush seals have emerged to be a very promising technology for sealing high pressure areas from low pressure areas such as those found in gas turbine engines. Indeed, there is a substantial reduction, e.g., an order of magnitude, in brush seal leakage flow over the present day technology of labyrinth seals, the main disadvantage associated with labyrinth seals being that the clearance of the seal around the shaft tends to increase appreciably over time due to shaft excursions and thermal growth. The resulting increase in parasitic leakage can cause as much as 17 percent loss in power and 7.5 percent increase in specific fuel consumption in the case of engines.
Typically, brush seals inhibit the flow of a gas (or liquid) in a stream along a shaft. The gas (or liquid) is sealed, for example, within a machine housing and has a system pressure. The area outside of the sealed machine housing toward which the sealed gas will tend to leak has a discharge pressure, the system pressure being greater than the discharge pressure.
Such a brush seal generally comprises an annular retaining plate, an annular back plate, and a plurality of bristles secured between the retaining plate and back plate. The bristles extend inwardly from the outer peripheral edges of the retaining plate such that their free ends run against the rotating shaft to thereby seal the system pressure from the discharge pressure.
While the system pressure acts primarily over the entire free length of the bristles, the discharge pressure acts over the overhang area of the bristles. An imbalance in these two axial forces results in the bristles loading against the back plate with a high mechanical contact force along the length of the bristles. It should be noted that the contact force will also be generated at the interfaces between the bristles by varying degree.
Frictional forces thus engendered by the contact force at various interfaces between the bristles and the back plate and between the bristles themselves give rise to bristle hysteresis with any radial movement of the shaft. For example, during a radial excursion of the shaft, the bristles are forced radially outward. Subsequently, when the shaft withdraws, the bristles do not drop back down on the shaft unless the differential between the system pressure and the discharge pressure is decreased to a small value. In fact, even a few psig pressure differential across the seal can prevent free radial movement of the bristles. This bristle hysteresis leaves a large gap between the bristles and the shaft, and hence, causes an appreciable increase in leakage along the shaft. Further, the leakage increase is likely to be cumulative every time a shaft excursion takes place in different directions.
Another detrimental effect of the above-described mechanical contact force is that it increases the radial stiffness of the bristles. Because the bristles are prevented from moving freely in the radial direction by the contact force, the effective stiffness of the bristles increases with rise in the differential between the system pressure and the discharge pressure. Typically, an order of magnitude increase in bristle stiffness is seen with a differential pressure rise of 60 psig from zero level. As a result of this bristle stiffening, the sliding interface between the bristles and the shaft experiences significantly higher contact load at a higher differential pressure for a given radial interference or an applied radial excursion of the shaft. This, in turn, results in accelerated wear of the bristles and degradation of the surface of the shaft.